1. Field of the Invention
The present invention relates to an optical scan apparatus having jitter amount measuring means. More particularly, the invention relates to an optical scanner with jitter amount measuring means capable of measuring scan unevenness, which could be caused by production errors of light deflector being a constituent of scan optical system and by drive errors of motor driving the light deflector, at a high precision in optically scanning a surface to be scanned, whereby the optical scanner can correct the scan unevenness to enable high precision optical scan, and, therefore, which may be suitably applicable for example to a scan optical system in a laser beam printer (LBP) or in an image reading apparatus.
2. Related Background Art
Conventionally, an optical beam (as also called as an optical flux) emitted from a laser source is optically modulated according to an image signal in a scan optical system used for example in LBP. The thus optically modulated beam passes for example through a collimator lens and a cylindrical lens, and is then deflected by a light deflector comprising a rotary polygon mirror. The deflected beam is converged through an f-.theta. lens system to form a spot on a surface of photosensitive recording medium, and the medium surface is optically scanned with the spot beam, whereby an image is recorded.
The scan optical system often has scan unevenness risen from a difference in scan time for each scan line and a scan position deviation in the main (horizontal) scan direction in optically scanning a surface to be scanned because of the manufacturing errors of light deflector being a constituent of the scan optical system and the drive errors of motor driving the light deflector, whereby a quality of image could be degraded. (The scan unevenness and the scan position deviation will be referred to as "jitter.")
There are various proposals of scan optical system which can optically scan a surface to be scanned at a high precision by measuring a jitter amount (amount of scan unevenness) by means of jitter amount measuring means.
An example is described in G. Toyen, "Generation of Precision Pixel Clock in Laser Printers and Scanners," SPIE, vol 84 (1976), in which two optical beams are used, one of which is used as a scanning light beam for optically scanning the surface to be scanned, and the other of which is as an optical beam for detecting a jitter amount (as a detecting light beam).
The detecting light beam is guided to enter a grating plate, which is disposed on a surface optically equivalent to the surface to be scanned and which has a plurality of grating lines arranged at predetermined intervals. The grating plate is optically scanned with the detecting light beam to obtain a pulse light signal. The pulse light signal from the grating plate is guided through a condensing means to a photo detector to produce a sychronizing signal. Then, the optical scan is carried out while controlling a light emission timing of the scan optical beam emitted from light source means for image recording, based on the synchronizing signal.
Another example is disclosed in Japanese Laid-open Patent Application No. 2-277018, in which an optical beam for detection is used for scan along a reference grating plate disposed at a position optically equivalent to a surface to be scanned with a scanning light beam so as to obtain a pulse light signal, the light signal is guided through an optical fiber bundle and a light waveguide member to a photo detector to produce a synchronizing signal, and the surface to be scanned is scanned based on the synchronizing signal to effect reading or recording of image or the like.
Still another example is disclosed in Japanese Laid-open Patent Application No. 2-106715, in which an optical beam is used for scan along a grating disposed on a plane optically equivalent to a surface to be scanned to obtain a pulse light signal, the light signal is guided to light detecting means comprised of a lens array and a plurality of light receiving elements to produce a synchronizing signal, and the surface to be scanned is scanned based on the synchronizing signal to effect image recording.
In the example as disclosed in the above Japanese Application, the reference pulse signal is obtained from the optical beam emitted from a semiconductor laser for synchronous detection, the reference pulse signal is input into a PLL (phase-locked loop) circuit, a light emission power of the optical beam from the semiconductor laser for synchronous detection is detected, and the light emitting power of the optical beam from the semiconductor laser is controlled by control means to obtain a better synchronizing signal.
In these conventional optical scanning apparatuses, the photo detector for producing the synchronizing signal needed numerous lenses and light receiving elements to pick up optical beams transmitted through the grating plate over the entire scan width. Such an arrangement increased the number of parts and made the entire apparatus complicated, making a reduction in cost of production difficult.
Further problems are such that adjustment of arrangement of such optical members is very difficult and that the entire apparatus is made larger in size and complex in arrangement.